Continuous monitoring of eccentricity and coaxial capacitance variables during electrical insulation extrusion

ABSTRACT

A system for monitoring telephone cable insulation and jacket wall thickness and centering during manufacture consists of four capacitive probe segments spaced at 90* intervals with a dielectric gap separating adjacent segments. The probe is flanked by guard tubes which along with each segment are connected to a switch capable of placing any or all segments at guard potential. In various configurations ordered by the switch, capacitance measurements are made to yield indicia of coaxial capacitance, eccentricity and ovality.

United States Patent 1191 Jones, Jr.

1 1 July 24, 1973 1 1 CONTINUOUS MONITORING OF ECCENTRICITY AND COAXIALCAPACITANCE VARIABLES DURING ELECTRICAL INSULATION EXTRUSION [75]Inventor: Charles Elmer Jones, Jr., Fairfield,

[52] US. Cl 324/61 R, 318/662, 324/54 [51] Int. Cl G011 27/26 [58] Fieldof Search 324/61 R, 54; 318/662; 323/93 3,096,478 7/1963 Brown 324/543,209,247 9/1965 Mead et a1... 324/61 R 3,355,664 11/1967 Franke 324/61R 3,466,391 9/1969 Ellis 324/61 R X I FOREIGN PATENTS OR APPLICATIONS1,052,321 9/1953 France 324/61 R Primary Exgzminer Stanley T.Krawczewicz Attorney- R. J. Guenther, W. L. Keefauver et a1.

[57] ABSTRACT A system for monitoring telephone cable insulation andjacket wall thickness and centering during manufacture consists of fourcapacitive probe segments spaced at 90 intervals with a dielectric gapseparating adjacent segments. The probe is flanked by guard tubes which[56] References Cited along with each segment are connected to a switchcav UNITED STATES PATENTS pable of placing any or all segments at guardpotential. 2,604,512 7/1952 Bacon et a1. 324/61 R i nous i 1 2 :39 i2,866,146 12/1958 Rodrigez, Jr... 318/662 x mefsuremen s 0 We 2,892,1526/1959 Buisson 324/61 R a1capacltanceeccenmclty and 2,729,213 1/1956Broekhuysen et al. 324/61 R X 2,765,441 /1956 1 Gambrill 324/61 R2,804,592 8/1957 Biskeborn 324/61 R 7 Claims, 4 Drawing Figures WIRESUPPLY IB HEAT EXTRUDER p s-nc Y 25 23 21 17 n Q 2e@ 28 I M I ADVANCINGADVANCING ROLLER ROLLER EXTRUDER WIRE I b E 206 CENTERING CONTROL T0SWITCH 30 X I. Y DIRECTION CONTINUOUS MONITORING OF ECCENTRICITY ANDCOAXIAL CAPACITANCE VARIABLES DURING ELECTRICAL INSULATION EXTRUSIONFIELD OF THE INVENTION BACKGROUND OF THE INVENTION a single wire, thethickness of the insulation as well as.

itsconcentricity about the underlying conductor can vary. The resultingvariation in coaxial capacitance may be unacceptably high for use ofthat insulated wire in digital transmission systems and high frequencyusages, such as video telephony. Furthermore, a wire that is notcentered in its insulation will exhibit a higher coaxial capacitancethan would be the case if the same thickness of insulation weremaintained concentrically about the wire. Also, the nonconcentricallyinsulated conductor when combined with a concentrically insulatedconductor creates high capacitance unbalance to ground, which again canbe unacceptable for the mentioned uses.

It is of course desirable to control as many of the cable manufacturingvariables as possible. To this end, capacitance monitors have been usedin the prior art to control coaxial capacitance of, for esample, aninsulated wire during extrusion. However, these capacitance monitors donot insofar as applicant is aware, provide a measurement of thecross-sectional eccentricity of the insulation about the wire. Theeccentricity typically is instead checked by cutting occasional sam plesof the production line wire and measuring the insulation wall thicknessvariations using an optical comparator. If this check indicatesadjustment is required, the extruder die is adjusted to center the wire.Of course, this technique is time consuming at best, and, in any case,does not provide continuous monitoring. Accordingly, one object of theinvention is to continuously monitor thewallthickness of plasticinsulation about a conductor during extrusion.

A further inventive object is to continuously monitor theconcentricity,eccentricity, or ovality as the case may be, of plasticinsulation with respect to the axis of the underlying wire.

A further inventive object is to monitor and adjust the capacitanceunbalance to ground during making of extruded pair or of extruded quad,without the necessity of isolating and switching the individualconductors within the insulation.

A further inventive object is to detect the concentricity, eccentricity,or ovality of extruded insulation during its manufacture, withoutphysically contacting the wire. I A still further inventive object is tomonitor the thickness and eccentricity of plastic jacket about a cablecore.

SUMMARY OF THE INVENTION The foregoing and further objects are achievedpursuant to the present invention by a capitance monitor that uses amultisegment capacitance probe with means for electrically isolating arelatively short cable section undergoing the measurement. Circuitrythat permits selecting of certain combinations of the segments, or allsegments, for the capacitance measurements, is connected between theprobes and the monitor indicators.

In general, the structure to be monitored is passed along a locus ofpoints equidistant from each of several probe segments which can, forexample, number 4 or 8. At both the input and output sides of the probe,the unit to be monitored is passed through a guard tube whichelectrically cuts the unit so that only the capacitance to ground of thelength of wire within the probe is measured.

In a specific embodiment, the probe consists of four segments spaced atintervals with a dielectric gap separating adjacent segments. The probeis flanked by guard tubes which along with each segment are connected toa switch capable of placing any or all segments at guard potential. Invarious configurations ordered by the switch, capacitance measurementsare made to yield indicia of coaxial capacitance, eccentricity andovality.

BRIEF DESCRIPTION OF THE DRAWING ments made possible with anillustrative four-segment probe;

FIG. 3 is a schematic side view of the four-segment probe and associatedapparatus; and- FIG. 4 is a circuit diagram of the probe inputs and acapacitance bridge.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS Asseen in FIGS. 1 and 3a capacitive probe designated l0 consists of four segments 11, 12, 13,and'l4 each embracing slightly less than 90 of arc. The four segmentsll-l4 are arranged so that the four interior surfaces 11a-14a constitutesegments of a cylinder of radius r with respect to an axis denoted 15. j

The adjacent segments 11-14 are electrically separated, for example, bydielectric gaps, such as gap 16 between segments l3 and. Thesegments11-14 are mounted in spaced, cylindrical relation by plural phenolicresin washers l7 and by phenolic resin end sleeves 21, 22. Each segmentis, for example, 2 inches to 6 inches in length long enough to obtainthe desired sensitivity but short enough to avoid being cumbersome.

Two nonsegmented hollow cylindrical metallic guards 23, 24 are supportedcoaxially with respect to the probe segments by the sleeves 21, 22 inconjunction with insulative end sleeves 2 5, 26, respectively.

Use of the probe 10 is illustrated by a wire 27 with plastic insulation28 moving in tension by action of advancing rollers 20a, 20b along thecommon axis 15 of the guards 23, 24 and the probe 10. The probe segmentinterior surfaces 1 la-l4a and the interior of guards 23,

are spaced a distance of the order inch 1 inch from the outer surface ofplastic insulation 28. Moving in the direction of arrow 29, the wire 27originates from a supply 18, and passes through extruder head 19 whichextrudes plastic insulation. Process steps such as cooling baths, etc.,are not shown. The insulated wire 27-28 could as well be any otherassemblage having a central axis and insulative covering located at somenominal position with respect to a central axis.

Electrical leads denoted A, B, C, D are connected from the respectiveprobe segments 11-14 to a switch 30, as seen in FIG. 4. Electrical leadsE likewise are connected from the guards to switch 30 which, as seen inFIG. 4, consists of center open single pole double throw switches 39a,39b, 39c, 39d, each capable of connecting its respective lead A-D to a2OKC oscillator 40, or alternatively to a capacitance bridge 41 over acommon lead 42.

Bridge monitor 41 is a conventional hybrid coil type direct impedancebridge and consists of transformer 52, whose center tapped primary 53 isused as the ratio arms of the bridge. The secondary winding 54 providesthe bridge output which is fed to amplifier 55. The two resistors 49, 51and variable resistor 50 provide the means to balance the conductance ofthe insulated wire.

being measured. Variable capacitor 44, switches 47, 48 and capacitors45, 46 are used in calibrating and zeroing the bridge. Variablecapacitor 43 provides the means to set the bridge for the desiredcapacitance.

The block diagram shown in FIG. 2 depicts one method of determining thecoaxial capacitance and eccentricity of an insulated wire using thesegmented probe. Two of the units 31 depicted in FIG. 2 as units 31a and31b and described above are connected to the electrical leads A, B, C, Dfrom the respective probe segments 11-14. The coaxial capacitance is thesum of the c'apacitances C C C C, shown in FIG. 1. With either bridgemonitor set at the desired coaxial capacitance using variable capacitor43, FIG. 4, the output of the monitor 31a will be proportional to anyerror between the desired and actual coaxial capacitance. By openingswitch 34 and closing switch 37 the error voltage is applied only tointegrator 38, which in turn drives the line speed servo 38a.

Eccentricity in the X direction is proportional to the differencebetween capacitances C, and C in FIG. 1. For this measurement probeleads B and D are switched to guard common lead E, probe lead A isconnected to monitor 31a and probe lead B is connected to monitor 31b.Switch 37 is open during eccentricity measurements so as not to effectline speed. With both monitors 31a, 31b set at the same desired valuethe difference in the two monitor signals as detected by differentialamplifier 33 is proportional to C minus C For this measurement switch 34applies the correction signal to the X-servo 35. Eccentricity in the Ydirection is accomplished in the same manner using the Y-servo 36 andthe appropriate probe lead connections.

The X-servo 35 and Y-servo 36 develop signals which are then applied inconventional manner to control the extruder head operation throughcentering control 19a, thus to control the eccentricity of the insulatedwire 27-28. The line speed servo 38a develops signals which are appliedto advancing rollers a, 20b, thus to control the capacitance of theinsulated wire 27-28. Table I below shows various positions of theswitches 30, 34 and 37 and the resulting measurements.

TABLE 1 Switch 34 Switch 37, inte X- X- Moriitor Monitor grator servoservo Measure- Guard 31a 31b 38 36 ment;

None." A, B, C, D Nonem. In Out... Out Coaxial capacitancc. B, D A COut. In Out... X-eecentricity. A, 0.... B D Out Out In Y-cccentricity.

It is apparent that the inventive process and apparatus so far describedcan be connected in numerous ways with a computer control system that inresponse to the capacitance measurements adjusts the extruder lineparameters such as line speed and die orientation.

In a further inventive embodiment, an eight-segment probe (not shown) isused to measure coaxial capacitance, X-eccentricity and Y-eccentricityor extruded pairs and quads; and also to provide ovality measurementsfor the standard insulated wire eccentricity and coaxial capacitancemeasurements.

It is to be understood that the embodiments described herein are merelyillustrative of the principles of the invention. Various modificationsmay be made thereto by persons skilled in the art without departing fromthe spirit and scope of the invention.

What is claimed is:

1. Apparatus for monitoring the capacitance across an insulative layerextruded over an axially advancing metallic member comprising:

a capacitive probe having plural mutually insulated segments, each saidsegment comprising an inner surface disposed along a locus of pointssubstantially equidistant from said member axis and in noncontactingrelation to said layer;

guard means enveloping said advancing member adjacent the inlet and theoutlet ends of said probe for establishing a reference electricalpotential;

means for maintaining said metallic member at ground potential;

means for selectively connecting each said segment individually to saidguard means thereby placing each segment so connected at said referencepotential;

and means for measuring the capacitance between each segment not soconnected and said metallic member.

2. Apparatus pursuant to claim 1 wherein said guard means comprisesfirst and second hollow metallic cylinders disposed coaxially with saidprobe segments and having their inner surfaces located along said locusof points.

3. Apparatus pursuant to claim 2, wherein the space between said layer,said cylinders and said inner surfaces of said segments constitutes adielectric medium.

4. Apparatus for monitoring the capacitance across an insulative layerplaced over an axially advancing metallic member, comprising:

means, including two spaced-apart metallic guards each enveloping saidadvancing insulated metallic member, for electrically isolating adiscrete portion of said member from the remainder of said member;

four capacitive plates located between said guards spaced symmetricallyabout said insulative layer and at equal distances therefrom;

a sinusoidally varying reference electrical signal;

means for connecting said isolating means and ones of said plates tosaid signal;

and means for measuring the capacitance between each of the remainingsaid plates and said metallic member.

5. Apparatus pursuant to claim 4, further comprising means formaintaining said advancing metallic member at ground potential.

6. Apparatus pursuant to claim 5, further comprising means for utilizingsaid capacitance measurements of claim 2.

7. Apparatus for continuously deriving indicia of coaxial capacitance ofan insulated conductor and also for deriving indicia of the extent ofcross-sectional eccentricity of said insulation about said conductor,comprising:

means for advancing said insulated conductor tautly through a measuringstation;

at least four capacitive plates comprising like cylindrical segmentssymmetrically disposed at said station along a locus of pointssubstantially equidistant from said advancing conductor;

a sinusoidally varying reference source of electrical potential;

guard means disposed on either side of said station and connected tosaid source and comprising first and second hollow metallic cylindersdisposed coaxially with said probe segments and having their innersurfaces located along said locus of points;

means for maintaining said advancing metallic member at groundpotential;

capacitive bridge means;

and means for selectively connecting each said segment individuallyeither to said source or to said bridge means whereby, by connecting allsaid segments to said bridge and none to said source said coaxialcapacitance is measured; and by connecting a first set of oppositelydisposed said segments to said source and a second set of oppositely disposed said segments to said bridge, a measure of said eccentricity isachieved.

1. Apparatus for monitoring the capacitance across an insulative layerextruded over an axially advancing metallic member comprising: acapacitive probe having plural mutually insulated segments, each saidsegment comprising an inner surface disposed along a locus of pointssubstantially equidistant from said member axis and in noncontactingrelation to said layer; guard means enveloping said advancing memberadjacent the inlet and the outlet ends of said probe for establishing areference electrical potential; means for maintaining said metallicmember at ground potential; means for selectively connecting each saidsegment individually to said guard means thereby placing each segment soconnected at said reference potential; and means for measuring thecapacitance between each segment not so connected and said metallicmember.
 2. Apparatus pursuant to claim 1 wherein said guard meanscomprises first and second hollow metallic cylinders disposed coaxiallywith said probe segments and having their inner surfaces located alongsaid locus of points.
 3. Apparatus pursuant to claim 2, wherein thespace between said layer, said cylinders and said inner surfaces of saidsegments constitutes a dielectric medium.
 4. Apparatus for monitoringthe capacitance across an insulative layer placed over an axiallyadvancing metallic member, comprising: means, including two spaced-apartmetallic guards each enveloping said advancing insulated metallicmember, for electrically isolating a discrete portion of said memberfrom the remainder of said member; four capacitive plates locatedbetween said guards spaced symmetrically about said insulative layer andat equal distances therefrom; a sinusoidally varying referenceelectrical signal; means for connecting said isolating means and ones ofsaid plates to said signal; and means for measuring the capacitancebetween each of the remaining said plates and said metallic member. 5.Apparatus pursuant to claim 4, further comprising means for maintainingsaid advancing metallic member at ground potential.
 6. Apparatuspursuant to claim 5, further comprising means for utilizing saidcapacitance measurements of claim
 2. 7. Apparatus for continuouslyderiving indicia of coaxial capacitance of an insulated conductor andalso for deriving indicia of the extent of cross-sectional eccentricityof said insulation about said conductor, comprising: means for advancingsaid insulated conductor tautly through a measuring station; at leastfour capacitive plates comprising like cylindrical segmentssymmetrically disposed at said station along a locus of pointssubstantially equidistant from said advancing conductor; a sinusoidallyvarying reference source of electrical potential; guard means disposedon either side of said station and connected to said source andcomprising first and second hollow metallic cylinders disposed coaxiallywith said probe segments and having their inner surfaces located alongsaid locus of points; means for maintaining said advancing metallicmember at ground potential; capacitive bridge means; and means forselectively connecting each said segment individually either to saidsource or to said bridge means whereby, by connecting all said segmentsto said bridge and none to said source said coaxial capacitance ismeasured; and by connecting a first set of oppositely disposed saidsegments to said source and a second set of oppositely disposed saidsegments to said bridge, a measure of said eccentricity is achieved.